1. Field of Invention
The present invention is directed to light processing methods and apparatus, and in particular to light processing methods and apparatus providing wavelength control.
2. Background
Many types of light processors for use in analyzing matter are known. Examples of such light processors include spectrometers. Spectroscopy is the study of the spectral characteristics of matter, and the use of such spectral characteristics to obtain qualitative and/or quantitative information about samples of matter (also referred to herein simply as samples). Conventional spectroscopic techniques utilize absorption spectra of matter or emission spectra of matter, as determined by the energy level structures of constituent atoms and molecules, to determine the presence and/or quantity of such atoms and molecules in the matter.
Instruments used to measure absorption spectra are commonly referred to as absorption spectrometers. In absorption spectrometers, information is obtained about the composition of a sample by projecting light onto or through a sample and observing the amount of the light that is absorbed by the sample as a function of wavelength. Because atoms and molecules have unique absorption spectra (also commonly referred to as spectral signatures), it is possible to determine the presence and/or quantity of constituents of the sample of matter. Typically, spectra measured by a spectrometer are divided into two or more wavelength components for analysis. As one of ordinary skill in the art would understand, use of the term “wavelength” herein, such as when referring to a wavelength of light that is detected or a wavelength of light from a source, refers to light of the indicated wavelength and light from a finite band of wavelengths around said wavelength, as may be determined by the laws of physics and/or conventional design practices. Also, the term “light” as used herein is used to refer to radiation of any suitable wavelength, not just visible light.
There exist numerous types of conventional absorption spectrometers. Conventional absorption spectrometers typically have the following features in common: a light source that covers a desired band of wavelengths from which spectral signatures are to be determined; a detector (comprising a single detector element or an array of detector elements) that is sensitive to light in the desired wavelength range; and optical componentry (e.g., a focusing element) that collects the light after it interacts with the sample of matter and directs the collected light onto the detector. Additionally, because information is present in the absorbed light as a function of incident wavelength, an apparatus providing wavelength selection is typically present to enable information as a function of wavelength to be detected. The path from a light source, to a sample of matter, and possibly further (e.g., onto the detector) is commonly referred to as a “sample pathway.”
Conventional absorption spectrometers may employ any of several different wavelength selection and detection techniques, for example: (a) using a monochromator to project wavelengths of light sequentially onto a sample, followed by collecting the wavelengths with a detector, so as to obtain wavelength absorption information as a function of time; (b) projecting multiple wavelengths of light from a broadband light source onto a sample simultaneously, dispersing the wavelengths comprising the light after it interacts with the sample (e.g., using a diffraction grating), and then projecting the light onto a detector array, such that each detector element in the array receives light of a selected wavelength; or (c) projecting light from a broadband source though each of a series of fixed optical filters in a sequential manner (e.g., by locating the filters on a motor-driven chopper wheel), so as to bring a sequence of wavelengths of light onto a sample sequentially, and capturing the light after interaction with the sample so as to obtain wavelength information as a function of time. Spectrometers of type (a) are referred to herein as monochromator-type spectrometers, those of type (b) are referred to herein as optical multichannel analyzers (OMA), and those of type (c) are referred to herein as non-dispersive (ND) spectrometers.
Prior to measuring a sample of matter, any of the above types of spectrometers is typically calibrated to account for detector and/or light source characteristics that may affect a measured output from a detector. For example, calibration may account for a detector having different sensitivities to different wavelengths of light, the individual detectors of an array of detectors having different sensitivities to the same wavelength of light, a source producing more light in a given portion of a spectrum than in another portion of the spectrum, and/or the light source output varying over time.
An example of a calibration technique is “empty cell” calibration, in which, prior to measurement, the sample pathway is traversed by light from a spectrometer's light source without a sample present. Such a reference measurement provides a baseline measurement for each wavelength to be measured. In such spectrometers, the baseline spectrum is stored, for example, in a processor, and during subsequent measurements, in which a sample is present, the baseline spectrum is compared with a spectrum measured with a sample present (i.e., a sample measurement) in an attempt to reduce the effects of the source and detector on a measured sample spectrum. A drawback of this technique is that the time period between the baseline measurement and a sample measurement can be relatively large and variations that occur during this time period (e.g., variations in source output) can be large.
More complex spectrometers have both a sample pathway and a “reference pathway,”and are referred as dual beam spectrometers. The reference pathway is designed to be the same as the sample pathway, except that the reference pathway has no sample present. In such spectrometers, a switching mirror may be operated to alternate between measuring light from the sample pathway and measuring light from the reference pathway such that the source and detector characteristics may be occasionally measured, to reduce the time period between reference measurements and sample measurements. A processor is then used to reduce the effects of variations in characteristics of the detector and/or the source by comparing measurements of light from the sample pathway with the baseline measurements in a manner similar to systems employing only a sample pathway, as described above. A limitation of this calibration technique is that, unless the reference pathway and sample pathway are identical (except for the presence of a sample), there can be a residual wavelength-dependent error present in measured spectra.
Some conventional spectrometers measure light from a sample by continuously receiving light from a sample to be measured (i.e., they “stare” at the sample), except for an occasional empty cell (baseline) measurement. A drawback of such spectrometers can be that they provide a low dynamic range for measuring changes resulting from absorption by constituent atoms and molecules. The low dynamic range arises because the amount of light absorbed by the constituents may be a small percentage of the light incident on a detector. Accordingly, all of the information about spectral signatures is contained in a small variation in a relatively large background signal.
Additional drawbacks of such spectrometers include that they are susceptible to errors arising from drift of both the detector and the optical source, 1/f noise and shot noise. Both sources of drift cause the outputs measured by the detector to vary over time. 1/f noise and shot noise are well known sources of noise. 1/f noise arises in systems which are designed to measure low frequency signals, and shot noise arises from the randomness of the arrival of photons at a detector and increases with the number of photon measured (i.e., intensity of the light measured) To overcome these problems, dual beam spectrometers are commonly operated as so-called AC spectrometers. For example, in an AC spectrometer a switching mirror alternately projects light from a reference pathway and a sample pathway onto a detector in a periodic manner, so as to produce an AC component in the detector output signal. By AC coupling an output of the detector for subsequent processing, the background signal that is common to the reference signal and sample signal may be removed such that the amplitude of the AC signal is proportional to the light absorbed by a sample in the sample pathway. Additionally, by alternating between detecting the sample pathway and the reference pathway it is possible to reduce the effects of detector drift and 1/f noise. However, while AC filtering improves the quality of the signal (e.g., by improving dynamic range), such systems do not compensate for any residual wavelength errors that may exist between a sample pathway and a reference pathway, as was described above. Furthermore AC-filtering does not remove the effects of shot noise because the intensity of the light measured is not reduced.
After absorption is measured using any of the above spectrometers, a determination of the amount of one or more constituents may be calculated. Determination of the amount of one or more selected constituents present in a sample is referred to as chemometrics. Conventionally, analysis to determine the amount of the constituents is performed using post processing of an output measured by a detector. The post processing typically includes correlating the absorption by the sample (measured as a function of wavelength) to a library of chemicals having known spectral signatures, for example, using a computer. A drawback of such conventional spectrometers is that the post processing to determine presence and/or amount of constituents in a sample can be computationally intensive and time consuming.